User's Manual

®
Model No. OS-8459 Experiment 10: Focal Length and Magnification of a Thin Lens
27
Experiment 10: Focal Length and
Magnification of a Thin Lens
Purpose
The purpose of this experiment is to determine the focal length of a thin lens, and to
measure the magnification for a certain combination of object and image distances.
Theory
For a thin lens:
(eq. 10.1)
where f is focal length, d
o
is the distance between the object and the lens, and d
i
is the
distance between the image and the lens. By measuring d
o
and d
i
the focal length can
be determined.
Magnification, M, is the ratio of image size to object size. If the image is inverted, M
is negative.
Part I: Object at Infinity
In this part, you will determine the focal length of the lens by making a single mea-
surement of d
i
with .
Procedure
1. Hold the lens in one hand and the screen in the other hand. Focus the image of a
distant bright object (such as a window or lamp across the room) on the screen.
2. Have your partner measure the distance from the lens to the screen. This is the
image distance, d
i
.
d
i
= _______________
Analysis
1. As d
o
approaches infinity, what does 1/d
o
approach?
Required Equipment from Beginning Optics System
Light Source
Bench
Converging lens of unknown focal length
1
1
Instructors: see note on page 46.
Screen
Other Equipment
Metric ruler
Optics Caliper (optional, for measuring image sizes), PASCO part OS-8468
1
f
---
1
d
o
-----
1
d
i
----+=
d
o
∞≅